Method of treating phototubes



March 10, 1959 I R. GAUTHIER 2,877,078

- METHOD OF TREATING PHOTOTUBES Filed April 13. 1954 2 Sheets-Sheet 2 IN VE N TOR ROBERT GAUTH/ER ATTORNEYS United States Paten 'Ofilice Patented Mar. 10, 1959 METHOD OF TREATING PHOTOTUBES Robert Gauthier, Verona, N. J., assignor to Allen B.

Du Mont Laboratories, Inc., Clifton, N. J., a corporation of Delaware This invention relates to improvements in 'phototubes and ".to a method of treating phototubes, particularly to difierential heating steps in stabilizing photomultiplier tubes.

Photomultiplier tubes are phototubes comprising elec-' tronmultipliers, which operate to multiply within thetube the'number of electronsemitted by a photosensitive'cathode within the tube. This operation depends on the characteristic ease with which electrons may befreed from certain materials, such as cesium antimony and silver magnesium. The former is used primarily as a photoeniissive material, while .both may be used as .a secondaryemissive material which coats the surfaces of electrodes, or dynodes, in the electron multiplier section of photomultiplier tubes.

It is known thata cesium antimony coating has a higher ratio :of'freed electrons .to impinging electrons (the multiplication ratio, or gain) over the corresponding ratio of dynodescoated with silver magnesium when used as the secondary-emissive coating of a dynode. However, this improvement in gain is obtained at theexpense of stability sincethe gain of silver magnesium dynodes is substantially constant over the life of the tube while the gain of a cesium antimony dynode decreases appreciably as the tube ages. Furthermore, the deleterious leakagecurrent of cesium "antimony dynode structures is greater than the leakage current-of silver magnesium dynode structures by a factor of as much as to 500:1. However, it is preferable to use,.ce sium rantimonyasa photo-.emissive material. Unfortunately though, when the desirable ,ces ium antimony photo-emissive surface 'is 'formed by vaporizing cesium, the material also condenses on the silver magnesium secondary .emissive layer of the dynodes, thus adversely affecting their secondary emission characteristics.

The. present invention provides amethod of increasing the gain of silver magnesium dynode structures without impairing the stability thereof. Broadly, this method consists in heating the dynode structure of such tubes to a temperature higher than the tube walls so as to evaporate from the dynode surfaces the cesium, which acts as a deleterious impurity. In addition to improving the gain of each dynode, this differential heating process increases the sensitivity of the photocathode because the cesium which is an undesirable impurity on the dynode surface is just the reverse on the photocathode, and by cooling as much as possible the portion of the tube wall on which the photocathode is located, the material evaporated from the dynode condenses on the relatively cool photocathode and improved its operation.

One object of this invention is to provide an improved phototube.

Other objects are to provide a photomultiplier tube using stable silver magnesium dynodes, and to provide a method for increasing the gain and cathode sensitivity of such tubes.

Other objects will be apparent from the following specification together with the drawings in which;

Fig. 1 is a side view of a phototube with parts, such as the glass wall, broken away to show the dynodes therewithin and certain of the dynodes also broken away to show the interior thereof, together with a heating coil wound about the envelope of the tube as used in one step during manufacture of the tube; and

Fig. 2 is a side view of the phototube, again with parts of the glass wall broken away to reveal the interior thereof, the view being at a different angle from that of Fig. 1, together with modified or alternative heating elements adjacent the envelope.

Fig. 1 shows a phototube comprising a bulb 11 having a faceplate 12 on which a photocathode 13 is deposited. The photocathode 13 is a composite, and the first layer, consisting of manganese and antimony, is deposited on the faceplate 12.

A dynode structure, indicated generally at 14,v is supported within the bulb 1-1 by means which in themselves are well known and, for the sake of clarity, are not shown. The dynode structure comprises dynodes 16-25, each of which has an electron multiplying surface coated with a substance such as silver magnesium, having stable electron multiplying characteristics. Within the last dynode 25 in the electron path, is an anode 26 in the form of a wire grid.

A shield 28 having an opening surrounding the electron entrance of dynode 16 is located at the upper end of the.

dynode structure 14, and a metallic coating band 29 on the sidewall of bulb 11 makes electrical contact with the photocathode 13. A pair of receptacles 31 and 32, containing-sources of cesium in the form of cesium pellets, is supported withinthe bulb -1-1 by any convenient means, such as by the dynodes 22 and 24. The invention is not limited by mechanical features of the phototube but is directed-toward chemical or physical aspects of materials used therein.

After the bulb 11 has been evacuated a coil 33 (one pair of such coils) is brought into position adjacent the wall of the bulb in a region near receptacles 31 and 32 and radio-frequency energy is supplied to the coil toheat. the receptacles and thereby volatilize the cesium pellets so as to fillthe bub 1-1 with cesium vapor. This vapor reacts with the previously deposited antimony on the faceplate portion of the bulb to form the second layer of the:

photocathode 1-3. 7

-It is only to form this requiredcesium'antimony photoemissive second layer that the cesium vapor is introduced into the tube. Unfortunately, the vapor condenses not only" on the photocathode area but also on-the dynode. structure '14 andon the silver magnesium coated surfaces of the dynodes. cesium are shown on the silver magnesium layer 36 as a result of condensation. Similar cesium droplets collect on corresponding surfaces of other dynodes, where their presence adversely afiects operation. These droplets are not removed by baking the phototube in an oven because heating merely re-evaporates the cesium droplets (both those shown as having condensed on the dynode surface as well as the relatively innocuous droplets 37 which have collected on the inner wall of bulb 11) and, when the phototube has cooled, droplets of cesium again condense on the silver magnesium surfaces of the dynode structure 14.

In order to remove the droplets 34 from the dynode structure and prevent their returning, the present invention includes the step of heating the phototube differentially to create more heat on the dynode structure 14 than on the wall of bulb 11. The cesium droplets 34 then evaporate and condense out on the cooler wall of bulb 11 or, preferably on the photocathode layer 13.

Any droplets 34 which may condense on the Wall of bulb 11 will not have a deleterious effect on the gain of For instance, in Fig. 1 droplets 34 of the dynodes 1625; and any droplets which condense on the photocathode 13 add more cesium thereon and sensitivity is improved.

This differential heating may be improved by cooling the faceplate 12 as much as possible so that there is a greater tendency for the cesium to condense on the photocathode 13 than on other parts of the wall of bulb 11.

The differential heating may be carried out by induction heating from radio-frequency energy supplied to a coil 38 surrounding the phototube, or by heating the coil 38 by electrical direct current and heating the dynode structure 14 by infra-red radiations. In either case, the metallic dynode structure 14 is heated more than the glass wall of bulb 11.

Another alternative is shown in Fig. 2 where an infrared lamp 39 is shown in position to radiate heat to the dynode structure 14. An air blast may be used to cool the faceplate 12, and following the differential heating process, the tube may be baked sufficiently to fix the cesium in position wherever it may have condensed.

By this improved heating process it is common to in crease the overall gain of the dynode structure by a factor of three; from a gain of about 80,000 times to a gain of about 250,000 times. Simultaneously, and as a desirable byproduct, the sensitivity of photocathode 13 may be raised from 40 microamperes per lumen to 55 microamperes per lumen, which is equivalent of raising the overall output of the tube by a factor of more than 3:1. It may be necessary to repeat the differential heating process more than once to achieve the gain and photocathode sensitivity of which the phototube is capable.

Although this invention has been described with reference to a specific embodiment, others will be apparent to those skilled in the art and the method may be used in the manufacture of other electron discharge devices.

What is claimed is:

1. In the manufacture of a phototube comprising a bulb having a glass Wall, a silver magnesium dynode structure, and a source of cesium, the method consisting in the steps of heating said source to create cesium vapor within said bulb; heating said dynode structure to a temperature greater than that of the wall of said bulb whereby cesium on said dynode structure is evaporated and condensed on said wall.

2. In the manufacture of a phototube comprising a bulb having a glass wall, a photocathode layer on a portion of the wall, a silver magnesium dynode structure, and a source of cesium, the method consisting in the steps of heating said source to create cesium vapor within said bulb; cooling said phototube to allow said vapor to settle on said photocathode layer; and differentially heating said phototube so that said dynode structure is at a temperature greater than that of the wall of said bulb where- 4 by cesium which has settled on said dynode structure is evaporated and condenses on said wall.

3. In the manufacture of a photomultiplier tube comprising a bulb having a glass wall, a photocathode layer on a portion of the wall, a silver magnesium dynode structure, and a source of cesium, the method consisting in the steps of heating said source to create cesium vapor within said bulb; cooling said phototube to allow said vapor to settle on said photocathode layer; differentially heating said phototube so that said dynode structure is at a tem-' perature greater than that of the Wall of said bulb whereby cesium which has settled on said dynode structure is evaporated again; and maintaining the temperature of said photocathode layer relatively cool to cause the cesium evaporated from said dynode structure to condense on said photocathode layer.

4. In the manufacture of a photomultiplier tube comprising a bulb having a glass wall, a combined photocathode layer composed of manganese layer on a portion of the wall of said bulb and a cesium antimony layer'on said manganese layer, a dynode structure having silver magnesium photomultiplier surfaces and a cesium pellet attached to said dynode structure, the method consisting in the steps of heating said pellet to createcesium vapor within said bulb; forcing said vapor to settle on said photocathode layer; differentially heating said phototubeso that said dynode structure is at a temperature greater, than-that of the wall of said bulb whereby cesium which has settled on said dynode structure is evaporated again; and maintaining the temperature of said photocathode layer relatively cool to allow the cesium evaporated from said dynode structure to settle on said photocathode layer;

- and baking said phototube to stabilize said cesium.

5. In the process of manufacturing photomultiplier tubes having silver magnesium secondary emissive surfaces, on the dynodes and a cesium antimony photo-emissive surface on the faceplate, and including the step of vaporizing cesium to form the photo-emissive surface whereby some cesium inadvertently condenses on the silver'magnesium dynode surfaces and adversely afiects the operation thereof, the improvement comprising the additional; step of heating said dynodes to a temperature high enough.

' to evaporate said cesium from said dynodes while simultaneously cooling said faceplate to encourage said evape' orated cesium to condense thereon.

References Cited in the file of this patent UNITED STATES PATENTS 1,927,812 Thomson Sept. 19, 1933 2,161,458 De Boer et al June 6, 1939 2,401,734 Janes June 11, 1946 2,574,356 Sommer Nov. 6, 1951 2,666,864 Longini Jan. 19, 1954 

1. IN THE MANUFACTURE OF A PHOTOTUBE COMPRISING A BULB HAVING A GLASS WALL, A SILVER MAGNESIUM DYNODE STRUCTURE, AND A SOURCE OF CESIUM, THE METHOD CONSISTING IN THE STEPS OF HEATING SAID SOURCE TO CREATE CESIUM VAPOR WITHIN SAID BULB; HEATING SAID DYNODE STRUCTURE TO A TEMPERATURE GREATER THAN THAT OF THE WALL OF SAID BULB WHEREBY CESIUM ON SAID DYNODE STRUCTURE IS EVAPORATED AND CONDENSED ON SAID WALL. 